Unit 1 Atomic Structure and Nuclear Chemistry Introduction to the - - PowerPoint PPT Presentation

Unit 1 – Atomic Structure and Nuclear Chemistry

Introduction to the atom

Modern Atomic Theory

 Atoms cannot be subdivided, created, or destroyed in ordinary chemical reactions. However, these changes CAN occur in nuclear reactions!  Every atom has different properties from other atoms  All matter is composed of atoms Ex: grinding down a gold ring

Modern Atomic Theory

Theory

• A set of tested hypotheses that gives an
• verall explanation of some natural

phenomenon. Ex: Cell theory & Evolutionary theory

Wait, it’s “only” a theory? Why are we learning it then?

• A theory is a powerful term in science

We can now see atoms …sort of

The Kanji characters for "atom." This image was formed by using the tiny tip of an STM to pick up individual atoms of iron and place them on a copper (111) surface. In 1981 a STM (Scanning Tunneling Microscope) was created.

• We can see them and

manipulate them.

Nanotechnology is coming

Atoms can be moved and molded to make various devices such as molecular motors

Structure of the Atom

Accessing Prior Knowledge

1. Based on your previous science classes, draw a generic atom and label where you’d find the nucleus, protons, neutrons, & electrons. 2. For a common beryllium atom, what is the: a) # protons? b) # neutrons? c) # electrons?

Structure of an Atom

Particle Charge Mass # Location Purpose Electron

• 1

Electron cloud Behavior of element Proton +1 1 Nucleus Identity of element Neutron 1 Nucleus Stability of nucleus

Nucleus (protons + neutrons) Electrons (in electron cloud)

1/2000th the mass of P+ & N

Charges in an Atom

The atom is generally neutral because: # of negative electrons = # of positive protons The nucleus is positively charged because: Contains positive protons (and neutrons which don’t have a charge).

The Atomic Scale… most pictures are really inaccurate!

• nucleus (protons and neutrons) is

small and dense and contains most

• f the mass of the atom.
• The electron cloud (where

electrons are found) contain most of the volume (3-D space) of an atom.

Not drawn to scale (electrons would be really far away) & the nucleus tiny.

A penny has 2.9 x 1022 atoms. Atoms are mostly empty space.

Atomic sizing… an analogy

…the nucleus would be the size

• f a grain of

sand. If you could make an atom as large as a football stadium… The nucleus is really tiny compared to the total size

• f the atom, but it’s never drawn that way.

(electron cloud takes up most of the volume)

Fun Fact…Quarks

• The particles that make

up protons and neutrons.

Using the Periodic Table to determining # or protons, neutrons, and electrons

Reading the periodic table

Atomic #= # of protons & # electrons

• Proton # = Unique to every

atom (serves as an atom’s identity)

• (atoms are neutral and + and –

charges must balance out)

Reading the periodic table

Atomic mass= the average mass of that atom

• Round the atomic mass (ex:12 )
• (electrons don’t weigh much so

aren’t included in mass #) Mass #- = # protons + # neutrons

• Ex. Not all carbon atoms have

the same mass so we have an average (see isotopes).

Practice

Atomic # Atomic Mass Mass # # protons # electrons # neutrons

17

Cl

35.45

17 35.45 35 17 17 18

Forces that hold an Atom Together

Forces that hold atoms together

Electromagnetic Force Nuclear Force

Keeps electrons near the nucleus Keeps nucleus from breaking apart

Electromagnetic Force

Protons (in the nucleus) & Electrons are attracted to each

• ther because of their opposite

charges  Idea is that opposite charges attract 

Nuclear Force

• The nuclear force is a 100 X’s stronger than

the electromagnetic force and acts like a “glue”

• the “strong force” (aka nuclear forces)
• vercomes the electromagnetic forces as long as

the protons are very close together

• Electromagnetic forces should cause

this nucleus to break apart because

• f all the protons repelling each other

(same charge), but it doesn’t ????

Joke…

Atoms vs. Elements vs. Molecules/Compounds

Elements, atoms, & molecules

atom A B C molecule Element Contains only one kind of atom (pure) D Molecule (bonded) & element (pure) 2 or more atoms bonded together

Isotopes

Isotopes

• atoms of the same element having different masses due

to different #’s of neutrons. (most have at least 2)

Isotope (in hyphen notation) Nucleus Hydrogen–1 (protium) Hydrogen-2 (deuterium) Hydrogen-3 (tritium)

The # indicates the mass number of the isotope (version) you are referring to.

• They’re all still Hydrogen

because they have 1 proton

3 Isotopes (versions) of Carbon

Isotope = Almost all of the elements have at least 2 different isotopes. Some have 4, 5, or even 10. *They are all carbon because they have 6 protons

Calculating average atomic mass

Can you guess which isotope is most common in nature?

• Carbon-12 because on the

periodic table carbon has an average atomic mass of 12.01.

Isotope Atomic Mass (amu) % Natural Abundance C - 12 12.00000 98.89 C - 13 13.00335 1.11

Practice with Isotopes

Atomic # Atomic Mass Mass # # protons # electrons # neutrons 6 14.00 (estimate based on mass #) 14 6 6 8

One way to show isotopes in writing: Ex: Carbon-14

Notations- another way to show isotopes

235 92U

Element symbol Mass of Isotope (p+ + no) Atomic # (# of p+)

Hyphen Notation Nuclear Notation Uranium-235

Some Isotopes are Radioactive

• Some isotopes of elements

are unstable (aka radioactive)

• Too many protons or neutrons

in a nucleus (ratio important)

• Large elements (#84 & up)

are radioactive

• Small ones can be radioactive

too (see H isotopes )

Isotopes of hydrogen H-3 is radioactive

• Intro. to Radioactivity & the

Band of Stability

What determines if a nucleus will be radioactive?

• The neutron to proton ratio in

the nucleus is an important part

• f stability.
• Small, stable atoms= 1 neutron

for every 1 proton

• Bigger, stable atoms = 1.5

neutron/ 1 proton.

• Neutrons aid to increase the

nuclear force in larger atoms

• A nucleus with 84 protons or

more will be radioactive regardless of how many neutrons it has (because of proton repulsion)

Band of stability graph- isotopes located on edge are radioactive

How to use the Band of Stability Graph

• Graph the number of

protons (x-axis) vs. number of neutrons (y- axis) for the atom.

• If your point. . .

– off the band of stability = atom does not exist in nature (too unstable) – at the edge of the band of stability = atom is unstable (radioactive). – on the band of stability = atom is stable (not radioactive).

Types of Radioactive Decay

(Alpha, Beta, & Gamma)

& Balancing Nuclear Equations

Radioactive Decay (summary)

• An unstable nucleus will emit particles of alpha,

beta, or gamma rays (aka radiation) to become a more stable element.

• Elements with Atomic # 84 or higher are radioactive no matter

how many neutrons they have. (nuclear force only works when protons are close)

• Proton to Neutron Ratio determines stability (see band of

stability graph)

Ex: Uranium --> radioactive particles + Lead

(unstable) (stable)

• This happens naturally & spontaneously

3 Types of Radioactive Decay

• There are 3 types of

particles that can be emitted from an unstable nucleus: – Alpha (α) particles – Beta (β) particles – Gamma (γ) particles

Alpha Decay-

• Symbol:
• Helium nucleus
• 2 protons & 2 neutrons
• Problem: the

nucleus has too many protons which cause excessive repulsion.

• Solution: In an attempt to reduce the repulsion

between protons, a Helium nucleus is emitted.

He

4 2

Beta Decay

• Symbol-
• an electron
• Problem: too many

neutrons causes instability.

• Solution: a neutron is split into a proton and an

electron.

• electron is then emitted at high speeds.
• Proton is kept

e

1

Gamma Decay- Electromagnetic Radiation

• Symbol:
• high energy
• Problem: the

nucleus is at too high an energy.

• Solution: The

nucleus falls down to a lower energy state and, in the process, emits a high energy photon known as a gamma particle. Usually accompanies alpha and beta radiation

Y

Penetration and Damage by types of Radiation

Alpha- thin barrier can stop (they are big and heavy and can’t travel very far) Beta- clothing, wood, or aluminum can stop. Gamma- highly penetrating

• when inhaled or

ingested can be dangerous.

• when inhaled or

ingested can be dangerous.

• Can penetrate deeply into

the body & alter DNA (cobalt-60 used for cancer treatment)

Summary of 3 types of radiation

Symbol Nuclear Notation Identity Damage Alpha

α

Helium nucleus Least penetrating Beta ß electron Gamma γ High energy Most penetrating

He

4 2

Y

e

1

Balancing Nuclear Reactions

Unstable element Radioactive particle More stable element Law of conservation of Matter= matter can neither be created nor destroyed (“what goes in must come out”)

Rn He Ra

222 86 4 2 226 88

Mass 226 = 4 + 222 (just add top) Protons 88 = 2 + 86 (add bottom)

Gamma is not usually shown in equation (no effect)

Y

½ Life & Calculations

Nuclear Decay of Uranium-238

½ Life & Radioactive Dating

• Half Life= Time it

takes for ½ of the atoms of a radioactive substance to decay into a stable isotope. Half-life Ex: Carbon-14 = 5730 years Uranium-235 = 704 million years

Radioactive Decay Graph

• This graph shows the

number of parent atoms remaining over time.

• The half-life is determined

by how many years it takes for ½ if the atoms to decay.

• There are 18 out of the
• riginal 36 parent atoms

after 3.9 years.

Radioactive Dating (w/ Carbon-14)

• 2 carbon isotopes are found in

living things: C-14 (a radioactive isotope) C-12 (more common)

• They are incorporated into living

things at a constant rate when they eat (1 in every trillion carbons is C-14). C-14 decays, but is constantly replaced.

• The ratio of C-14: C-12 is constant

while an organism is alive & is the same for every organism.

• When an organism dies the C-12

remains the same, but amount of C-14 decreases (decays) at predictable rates.

Solving a ½ life Problem

A 100 grams of a radioactive substance has a ½ life of 10

• years. How many grams are left after 30 years?

100 g  50 g  25 g  12.5 grams Solving it mathematically: Y=A(1/2) t/h (100)(1/2) 3 = 12.5g

y= final amount A=staring amount T=time H= half life 10 yrs 10 yrs 10 yrs

Sample Problem

• The half-life of K-42 is 12.4 hours. How much
• f a 750g sample is left after 62 hours?

Solution: 62 hours/ 12.4 hour = 5 half-lives have gone by. 750g x .5 x.5 x .5 x .5 x .5 (each .5 is a ½ life) Answer: 23.4 g

Other types of Nuclear Reactions: Fission & Fusion

Both processes require extraordinary conditions to happen, and do not occur naturally on Earth

Nuclear Fission

• large nucleus is split into

two or more smaller nuclei

(process sped up by hitting it with a neutron)

• Releases Alpha, Beta,

Gamma Rays and a lot of energy

• used to power nuclear

weapons (atomic bombs), nuclear subs, & nuclear power plants

Nuclear Fusion

• 2 small nuclei smash into each
• ther forming a larger, more

stable nucleus.

• Uses: How our sun produces energy & how hydrogen

bombs work

• Pros: Release more energy than

fission & cleaner than fission (little radioactive waste)

• Cons: Takes a tremendous amount
• f heat and pressure to get atoms

to combine (no technology available yet)

Chemical Vs. Nuclear Reactions

Chemical Rxn Nuclear Rxn

• Atoms rearrange to form

new substances (atoms identities do not change)

• changing of the atoms nucleus (and

thus, the atom’s identity)

• Deals with small amounts of

energy

• Large amounts of energy released.

(1 million x’s more than chemical rxns)

• See mass defect (E= mc2)

Ex: burning of gas CH4 + O2 CO2 +H2O Ex: Fission, Fusion, & radioactive decay.

Mass Defect- the mass of an atom is

less than the sum of its parts!

• Mass of a Helium atom has been

mathematically calculated to be: 2 p+: (2 x 1.007276amu)= 2.014552 amu 2 N: (2 x 1.008666 amu)= 2.017330 amu 2 e: (2 x 0.0005486 “) = 0.001097 amu Total mass: 4.032979 amu

• The actual measured mass of the He atom put together is:

4.00260 amu

• Why is there a loss in mass?

(mass defect)

• The mass lost during the formation of the atom was

converted into energy to help hold the nucleus together.

Explaining mass defect: E= mc2

(Energy = mass x speed of light squared)

• E=mc2 says that mass can be

converted into energy when the nucleus is formed or changed.

• In nuclear reactions, large

amounts of energy are released when the nucleus changes.

• Energy is so large because c2

is speed of light2 and is a huge # ( c = 299,792,458 m/s)

Discovery of the Atom

Discovery of the Electron

In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle (the electron). Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.

Rutherford’s Gold Foil Experiment

 Alpha particles are helium nuclei  Particles were fired at a thin sheet of gold foil  Particle hits on the detecting screen (film) are recorded

Rutherford’s Findings

 The nucleus is small  The nucleus is dense  The nucleus is positively charged  Most of the particles passed right through  A few particles were deflected  VERY FEW were greatly deflected “Like howitzer shells bouncing off

• f tissue paper!”

Conclusions:

Practical Uses of Nuclear Chemistry

Radiation is a natural phenomenon

We are exposed frequently to sources of radiation (most naturally) In fact, you emit radiation from K-40 inside you.

Mass Spectroscopy

Machine that allows for the separation of atoms based on mass.

Readout On a Mass Spectra

Important Uses of Radioactive Isotopes- Bone Scans

• Patient is injected with a

radioactive isotope (Tc-99) that is attached to another molecule (phosphate).

• This molecule with a

radioactive tag travels through the body and accumulates in areas that bone growth is high (injuries).

• A special scanner picks up
• n the gamma rays being

emitted by Tc-99

Uses of Gamma Radiation

• Because of it’s high

frequency and penetrating power, gamma is useful in: – sterilization of medical equipment by killing bacteria – used to kill bacteria and insects in foodstuffs, particularly meat, marshmallows, pies, eggs, and vegetables, to maintain freshness “Gamma Knife”- Brain tumors are hit with gamma rays in this device.

“Atom Smashers”

The Large Hadron Collider (LHC) is a particle accelerator located at CERN, near Geneva,

• Switzerland. It lies in a tunnel under France and

Switzerland.

• particles are

accelerated to high speeds & collided with target atoms.

• resulting pieces from

the collision, as well as emitted radiation, are detected and analyzed.

• Can learn about the

particles that make up the atom and the forces that hold the atom together.

Joke

• A neutron goes into the pub and asks for a pint of

beer.

• "How much is that?" he asks the barman.
• The barman replies ...."For you, no charge."

Joke

• Proton runs into a bar and claims he just saw

big foot run by.

• Bar man ask, “are you sure?”
• Proton says, “I’m positive”

Joke

• Silver and copper are @ the bar when gold walks

in.

• They scream @ gold, " Au- You don't belong here."

joke

• Why do chemists call helium, curium and barium

the medical elements?

• A: Because if you can't helium or curium, you

barium!